Computerized apparatus for producing cross-section images of the body by the method of X-ray axial tomography are known, for example from U.S. Pat. Nos. 3,778,614 and 3,866,047 to Hounsfield. In one form of such apparatus a divergent beam of X-ray photons is directed from an X-ray source in a plane which passes through a body undergoing examination and thereafter impinges on an array of X-ray detectors which lie in the plane of examination. The X-ray source and detector array rotate (and in some embodiments also translate) about the body to produce a series of one-dimensional X-ray shadowgraphs which are combined in a digital computer, using well-known computational algorithms, to yield cross-section images of the examination plane.
Common X-ray sources, that is X-ray tube anodes or radioisotope sources, generally produce X-ray beams which diverge through substantial solid angles. In computerized axial tomography (CAT) equipment mechanical collimation is generally utilized in conjunction with the X-ray source to limit the divergence of the X-ray beam to a wedge or fan-shaped swath which is confined to the examination plane and to the included angle of the detector array. Unnecessary radiation dose to the patient and system noise from scattered X-ray photons is thereby reduced. The mechanical constraints of CAT equipment generally require that such X-ray source collimators have minimum weight, to permit rapid motion, and minimum length, to reduce the overall size of the rotating components and maximize the X-ray flux at the detector array.
Means are generally provided for adjusting the thickness of the X-ray beam, and thus the examination plane, in CAT apparatus. In prior art collimators, which generally comprised one or more long channels through bodies of X-ray absorbing material, two sets of moveable jaws were generally utilized to control the thickness of the X-ray swath. Such multiple jaw sets were required to limit the X-ray penumbra which would otherwise be produced if a single set of jaws were utilized with an X-ray source of finite dimensions.